Semiactive dampers, and in some instances control policies for them, are disclosed in U.S. Pat. Nos. 3,807,678, 3,995,883, 4,468,050, 4,468,739, 4,491,207, 4,696,489, and 4,742,998; as well as in U.S. Pat. application Ser. Nos. 06/913,067, filed 29 September 1986, and 06/945,380, filed 22 December 1986, both of which are owned by the assignee of the present application, and in an article by M. J. Crosby et al, entitled "VIBRATION CONTROL USING SEMIACTIVE FORCE GENERATORS."
Semiactive dampers may be of the "off/on" type or of the "continuously" or "infinitely" variable type. A damper of the first type is switched, in accordance with the dictates of a suitable control policy, between alternative "on" and "off" damping states or conditions. In its on state, the damping coefficient of the damper is of preselected relatively high magnitude. The term "damping coefficient," as used herein, means the relationship of the damping force generated by the damper to the relative velocity across the damper, which relationship is not necessarily linear. In its off state, the damping coefficient of the damper is of relatively low magnitude. This may be approximately zero, but in many vehicle suspensions should be of a magnitude sufficiently greater than zero as to discourage "wheel hop." A continuously variable semiactive damper is also switched during operation between an off state, wherein its damping coefficient is approximately zero or of other low magnitude, and an on state. However, when a continuously variable damper is in its on state the damping coefficient thereof may be and normally is changed between a large (theoretically infinite) number of different magnitudes. If operated pursuant to a suitable control policy, a continuously variable semiactive damper may be caused to perform, when in its on state, in a manner similar to the hypothetical "sky-hook" damper discussed in the hereinbefore-mentioned Crosby et al article and in U.S. Pat. No. 4,742,998.
A known control policy for a continuously variable semiactive damper dictates that damper be "on," and that the significant damping forces generated by it be proportional (although not necessarily linearly) to the absolute velocity of the supported member, when the sign of the product of such absolute velocity times the relative velocity between the supported member and the supporting member is positive, i.e., is greater than zero. The policy dictates that the damper be in its off state, wherein the damping coefficient is of preselected low magnitude, when the sign of the aforesaid product is negative, i.e., when the product is less than zero. Generally comparable results may be achieved, particularly at relatively high frequency excitations, by use of an alternative control policy which dictates that damping forces proportional to the relative displacement between the supported and supporting members be produced by the continuously variable semiactive damper at those times when the product of the relative velocity times the relative displacement between the members is less than zero, i.e., when the sign of the product is negative or minus; and that the damping forces be of low magnitude when the aforesaid product is greater than zero, i.e., when its sign is positive or plus.
Although generally producing good results, vibration attenuating systems having continuously variable semiactive damper means controlled in strict accordance with the control policies of the foregoing or similar types may experience shock forces of significant magnitude at some of the times when the damper is switched between its different damping states or conditions. The aforesaid shocks may stress system components to such an extent as to shorten their useful life, and/or may cause the generation of objectionable noise. The problem of noise generation may be particularly apparent in automobile suspensions or other systems containing a resilient deformable member, such as an automobile tire, that is capable of storing energy upon deformation, and of abruptly releasing its stored energy when allowed to rapidly return toward an undeformed condition.